[PATCH] powerpc/boot: Fix build failure since GCC 4.9 removal

[Michael Ellerman <mpe@ellerman.id.au>]

Stephen reported that the build was broken since commit
6d2ef226f2f1 ("compiler_attributes.h: drop __has_attribute() support for
gcc4"), with errors such as:

  include/linux/compiler_attributes.h:296:5: warning: "__has_attribute" is not defined, evaluates to 0 [-Wundef]
    296 | #if __has_attribute(__warning__)
        |     ^~~~~~~~~~~~~~~
  make[2]: *** [arch/powerpc/boot/Makefile:225: arch/powerpc/boot/crt0.o] Error 1

But we expect __has_attribute() to always be defined now that we've
stopped using GCC 4.

Linus debugged it to the point of reading the GCC sources, and noticing
that the problem is that __has_attribute() is not defined when
preprocessing assembly files, which is what we're doing here.

Our assembly files don't include, or need, compiler_attributes.h, but
they are getting it unconditionally from the -include in BOOT_CFLAGS,
which is then added in its entirety to BOOT_AFLAGS.

That -include was added in commit 77433830ed16 ("powerpc: boot: include
compiler_attributes.h") so that we'd have "fallthrough" and other
attributes defined for the C files in arch/powerpc/boot. But it's not
needed for assembly files.

The minimal fix is to move the addition to BOOT_CFLAGS of -include
compiler_attributes.h until after we've copied BOOT_CFLAGS into
BOOT_AFLAGS. That avoids including compiler_attributes.h for asm files,
but makes no other change to BOOT_CFLAGS or BOOT_AFLAGS.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Debugged-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
---
 arch/powerpc/boot/Makefile | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)


This seemed safer as a minimal fix, rather than doing a more
comprehensive separation of CFLAGS/AFLAGS. We can do that in a future
patch.

It passed my usual build/boot tests, including booting the built zImage
on some real hardware, so this is good to go from my POV.

cheers

diff --git a/arch/powerpc/boot/Makefile b/arch/powerpc/boot/Makefile
index 6900d0ac2421..089ee3ea55c8 100644
--- a/arch/powerpc/boot/Makefile
+++ b/arch/powerpc/boot/Makefile
@@ -35,7 +35,6 @@ endif
 BOOTCFLAGS    := -Wall -Wundef -Wstrict-prototypes -Wno-trigraphs \
 		 -fno-strict-aliasing -O2 -msoft-float -mno-altivec -mno-vsx \
 		 -pipe -fomit-frame-pointer -fno-builtin -fPIC -nostdinc \
-		 -include $(srctree)/include/linux/compiler_attributes.h \
 		 $(LINUXINCLUDE)
 
 ifdef CONFIG_PPC64_BOOT_WRAPPER
@@ -70,6 +69,7 @@ ifeq ($(call cc-option-yn, -fstack-protector),y)
 BOOTCFLAGS	+= -fno-stack-protector
 endif
 
+BOOTCFLAGS	+= -include $(srctree)/include/linux/compiler_attributes.h
 BOOTCFLAGS	+= -I$(objtree)/$(obj) -I$(srctree)/$(obj)
 
 DTC_FLAGS	?= -p 1024
-- 
2.25.1